Hello there, this is my first ever thread and I wanted to pick your collective horse brains. Sorry it's a bit long.
I have a very elderly Connemara (between 30 and 35 years old) who has been fully retired for 5 years. He is arthritic in his back end, has a couple of cataracts and a few nodules on his dock, but otherwise in rude health (literally rude, he oozes attitude from all his pores).
He lives out all summer and has recently come in for the winter. I try to turn him out most days for exercise if the weather is not too foul. Today it was a balmy 9 degrees so I let him out without his rug so he could scratch at the massive, shaggy winter coat. He spent the day bumbling about but, according to the stable owner he lay down for a lot of the day. As I was mucking him out he was watching from the field when he suddenly disappeared from view and when I went to check he was lying flat out on the ground. I called him a few times but there was no reaction. Thinking he was gone I ran over and he gave me a filthy look and stood up.
He is not agitated, not kicking at his gut, pacing, salivating etc. I know that colic is still a possibility, as is him being just plain tired and old, but it was wondering if this is maybe a warning sign of the end approaching. 
. It is not usual for him to lie flat out unless there is some serious sunshine for him to bask in.
I am keeping a close eye on him for now but I think I may need to brace myself.
Has anyone any experience of similar?